c
\(\Delta \,V\,\,=\,\,6\,-\,2\,\,=\,\,4\,\,lit.\,\,P\,\,=\,\,1\,atm\)
\(W\,\,=\,\,-4\,L\,-atm\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1\,\,atm\,\,-\,\,litre\,\,=\,\,101\,\,joule\,\,\,\,\Rightarrow \,\,W\,\,=\,\,-\,\,405.05\,\,J\,\,\)
\(q\,\,=\,\,\Delta \,U\,-\,\,W\,\,\,\,\Rightarrow \,\,\,800\,\,=\,\,\Delta \,U\,\,+\,\,405.05\,\,\,\,\,\therefore \,\,\,\Delta \,U\,\,=\,\,394.95\,J\)